Depth Profiling of Ultra Thin Film Using Laser Induced Fluorescence Spectroscopy under Normal Incidence Conditions Based on the Reciprocal Principle

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Abstract
Depth profiling for thin film sample using laser induced fluorescence spectroscopy under normal incidence conditions, based on the principle of reciprocity, was studied. A laminated Langmuir–Blodgett model film sample (187 nm in thickness) with a concentration depth profile which varies as a stair-like function with 20.8 nm steps was prepared. Using this model film sample, observation angle dependence of fluorescence intensity was compared with theoretical results based on the principle of reciprocity, and it was shown that the principle held for the thin film sample having a continuous concentration depth profile. Secondly, these data were used to reconstruct the concentration depth profile by the least square method. The quantitative concentration depth profiling results obtained for the model film sample were satisfactory. This method can be used for film samples thinner than the wavelength of the excitation light and its spatial resolution in the depth direction is estimated to be in the order of a nanometer.